The sarcolemma (plasma membrane) of myocardial cells has high levels of Na-Ca exchange activity. Na-Ca exchange is an important mechanism for regulating myocardial Ca and thereby also regulates myocardial contractility. The objective of this project is to use molecular biological techniques to learn more about the role of the Na-Ca exchanger. Towards this goal, the specific aims of this project are as follows: 1. Genetic approaches to exchanger physiology. The level of the mouse cardiac Na-Ca exchanger will be manipulated using genetic approaches to overexpress or knockout the exchanger. Initial data indicate that overexpression of the cardiac Na-Ca exchanger makes the myocardium susceptible to hypertrophy and heart failure. Also, initial experiments with knockout mice indicate that embryonic heart tubes maintain excitation-contraction coupling and contract in the absence of the exchanger. The underlying hypothesis is that, by selectively altering the level of the exchanger, its role in cardiac physiology and pathophysiology will become more clear. 2. The exchanger gene family. There are many members of the Na-Ca exchanger gene family. Attempts will be made to identify the function of a putative exchanger clone that may represent a mitochondrial Na-Ca exchanger. Also, efforts will be made to determine the functions of putative exchangers identified in bacterial genome sequencing projects. These exchangers may represent Ca-H exchangers. The hypothesis is that information from homologous proteins will help us to better understand the sarcolemmal Na-Ca exchanger. 3. Interacting proteins and regulation of the exchanger. Initial data from this laboratory indicate that the Na-Ca exchanger interacts with and is regulated by calmodulin and, surprisingly, troponin C. The aim is to better understand the mechanisms and relevance of these novel interactions. The hypothesis is that interactions of the exchanger with calmodulin and troponin C have important physiological functions.